Voltage comparator

ABSTRACT

A voltage comparator with comparing means that are energized upon the occurrence of a clock-edge and that are de-energized when the comparison operation is completed. The comparing means are preceded by a voltage divider (D) for dividing the voltage to be compared and a switch (S 8 ) in series with the voltage divider for preventing current flow through the voltage divider when the comparing means are de-energized.

The invention relates to a voltage comparator for comparing an input voltage with a reference voltage, said voltage comparator comprising comparing means and clock-signal controlled switching means connected to said comparing means for energizing the comparing means upon a transition of the clock-signal and means for determining the completion of a comparison and for controlling the switching means for switching off the energizing of the comparing means when the completion of a comparison is determined. A voltage comparator of this kind is known from European patent application 1.026.826.

Such comparators are of particular interest for monitoring the battery of mobile products. In such products it is of primary importance that on the one hand the battery charge is monitored with sufficient accuracy and on the other hand the voltage comparator itself draws as less current as possible from the battery. One way to minimize the consumption of the comparator is to operate the comparator with reduced bias current, however this goes at the cost of the accuracy of the measurement. Another way is to operate the comparator intermittently. This may e.g. be done by activating the comparator only on “high” states of a clock signal of 1 KHz with small duty cycle, so that the actual comparison operation takes place only during a small portion of the 1 mS clock period.

A drawback of this method is that, when the said portion is too small, the comparator is switched off when the comparison is not yet finished, so that an incorrect result is obtained. On the other hand, when the portion during which the comparator is active, is too high the power consumption of the comparator is unnecessarily high. To overcome this, the prior art voltage comparator comprises means to determine the instant of completion of the comparison and to switch the comparator off when this instant has arrived.

Conveniently, in the voltage comparators of the above described kind, the reference voltage is obtained by an arrangement which is known in the art as a “bandgap”-voltage generator. Such reference voltage is about equal to 1.2 Volt and is substantially temperature independent. However, this means that input voltages can only be compared with 1.2 Volt. The present invention seeks to avoid this limitation while still a low power consumption of the voltage comparator is maintained and the voltage comparator of the invention is therefore characterized by a voltage divider for dividing the input voltage to be compared, the switching means comprising a switch connected in series with the voltage divider for inhibiting the flow of current through the voltage divider when the comparison means are de-energized. In this way, the power consumption of the voltage divider can be minimized to only a small fraction of the power consumption of an un-switched voltage divider. The lower power consumption also allows using a voltage divider of lower resistance with the advantage that lesser die area is occupied by the voltage divider.

The voltage comparator according to the present invention may be further characterized by said switching means comprising a Schmitt Trigger having an input which is controlled by the signal that controls said switch connected in series with the voltage divider, and whose output controls the energizing and de-energizing of the comparing means. The purpose of this Schmitt Trigger is to ensure that the said switch is fully “on” and consequently its impedance is sufficiently low, before the comparing means become operative.

The voltage comparator of the present invention may still further be characterized by said comparing means having two substantially identical outputs, said switching means being arranged to force different voltages to said outputs when the comparing means are de-energized and an exclusive OR gate connected to said two outputs for determining the completion of a comparison when the two output voltages are substantially equal and for controlling, through a clock controlled latch, the de-energizing of the comparing means and the switching off of the voltage divider. This arrangement presents a simple and reliable means for determining the conclusion of the comparing operation.

The invention will be described with reference to the accompanying figure, which shows a synoptic circuit diagram of a voltage comparator according to the invention.

The voltage comparator of this figure comprises two NMOS transistor T₁ and T₂ whose source electrodes are interconnected to a common current source I. The gate electrode of the transistor T₁ is connected to a voltage to be sensed V_(s) and the gate electrode of the transistor T₂ is connected to a reference voltage V_(r). The drain current of transistor T₁ is mirrored through a first current mirror of PMOS transistors T₃, T₄, followed by a second current mirror of NMOS transistors T₅, T₆ and a third current mirror of said transistor T₅ and an NMOS transistor T₇. The drain current of transistor T₂ is mirrored through a fourth current mirror of PMOS transistors T₈, T₉ and a fifth current mirror of said transistor T₈ and a PMOS transistor T₁₀. The drain electrodes of the transistors T₆ and T₉ are interconnected to constitute an output node N₁ and the drain electrodes of transistors T₇ and T₁₀ are interconnected to constitute a second output node N₂. This latter output node is coupled, to an input d of a clocked latch L₁ with clock input c and output terminal q. This output terminal q is coupled to an output O of the voltage comparator. It may be observed that the nodes N₁ and N₂ constitute the output terminals of two output stages, which are, apart from the switches S₁ and S₂, identical in construction and whose output voltages consequently will end up at the same value. When this situation occurs the comparison may be considered as being finalized and the comparator may safely be cut off.

The nodes N₁ and N₂ are each connected through a Schmitt trigger M₁ and M₂ respectively to an input of a XOR-gate X. The output of the XOR gate X resets a second clock-controlled latch L₂, through a third Schmitt trigger M₃ and an inverter W. Equally as the latch L₁, also the latch L₂ comprises an input terminal d, an output terminal q, a clock input c and moreover a reset input r. The input terminal d is connected to the positive supply voltage. When a rising edge appears at the clock input c, the latch will transmit the positive voltage at the input d to its output q and when the reset input r receives a “high” from the inverter W the output q of the latch will return to “low”.

The voltage comparator of FIG. 1 further comprises switches S₁, S₂, S₃ and S₄ that are controlled by the output q of the latch L₂ through a fourth Schmitt trigger M₄. The connections between this Schmitt Trigger and the switches are not shown in the figure. The switch S₁ connects the node N₁ to the positive supply when the latch L₂ is reset and the switch S₂ connects the node N₂ to the negative supply when this latch L₂ is reset. Therefore, one of the inputs of the XOR-gate X is “high” and the other input is “low” making the output of the XOR-gate “high”. When the latch L₂ is reset, the switch S₃ disconnects the interconnected source electrodes of the comparator transistors T₁ and T₂ from the current source I and the switch S₄ connects the gate electrodes of the current mirror transistors T₅, T₆ and T₇ to the negative supply

In operation, when a rising edge appears at the clock input c of the latch L₂, its output q will, through the Schmitt trigger M₄, open the switches S₁, S₂ and S₄ and close the switch S₃, as shown in the figure. This will put the comparator into operation and the two nodes N₁ and N₂ will not be stuck anymore to the positive and negative supply voltages respectively. When the voltage to be sensed V_(s) is higher than the reference voltage V_(r) the drain current of T₁, mirrored to T₆, will pull the voltage of the node N₁ down until this voltage is equal to the voltage of node N₂. On the other hand, when the voltage V_(s) is lower than the reference voltage V_(r), the drain current of T₂, mirrored to T₁₀ will push the voltage of the node N₂ up until this voltage is equal to the voltage of node N₁. In both cases the end result is that the two node voltages are equal, so that the output of the XOR-gate will fall to “low”. This will through the Schmitt trigger M₃ give a “high” at the output of the inverter W. This “high” will reset the latch L₂, the output of this latch becomes “low” and this will deactivate the comparator circuit until a new rising edge of the clock input appears. Consequently, when the comparison operation is completed the comparator will be shut off and any further power consumption is stopped. A standard comparator draws in operation a bias current that is typically 2 μA. The comparison time is 5 μs in the worst case. When the comparison is performed every 1000 μs, then the average consumption of the comparator is reduced to 2 μA/200=10 nA. It may be noted that the Schmitt Triggers, the XOR gate and the latches in CMOS technology, just like other gates in this technology, only draw a very short peak of current during the transition from “Low” to “High” and vice versa.

As explained in the introduction, the reference voltage V_(r) often has a fixed value of 1.2V. In order to be able to compare at a plurality of voltage levels, the voltage comparator of FIG. 1 comprises a voltage divider D, which has a plurality of series connected resistors R₁, R₂, R₃, R₄, R₅ and switches S₅, S₆ and S₇ bridging the resistors R₂, R₃ and R₄ respectively. Moreover in series with the voltage divider is a switch S₈, connected between the resistor R₅ and the negative supply terminal (ground). The voltage V_(in), whose value has to be monitored by the voltage comparator, is applied to the resistor R₁ and the voltage V_(s) that is applied to the gate electrode of the transistor T₁, is derived from the interconnection of the resistors R₄ and R₅. A control unit U controls the position of the switches S₅, S₆ and S₇ so that several comparison levels can be set. Of course, when only one comparison level is required, the switches S₅, S₆ and S₇ can be dispensed with and one single resistor can replace the resistors R₁ to R₄.

The switch S₈ is controlled by the output of the latch L₂, substantially simultaneously with the switches S₁ to S₄. Thus, when the comparator is switched off from the supply, also current through the voltage divider is stopped so that the battery to be monitored is no longer loaded. This is a very important feature of the comparator shown in the figure. With a 300 kΩ voltage divider and a voltage V_(in) to be monitored of 3 Volt the current through the voltage divider is 10 μA i.e. substantially more than the operational current of the comparator. By switching off the voltage divider substantially simultaneously with the switching off of the comparator the mean current through the voltage divider is reduced by the same factor (e.g. 200) as the reduction of the mean current of the comparator. The resistance value of the voltage divider can also be reduced, e.g. from 300 kΩ to 100 kΩ. to decrease the die area Resistor dividers are among the components that are most responsible for the consumption of die area in the IC. The switching of the voltage divider enables thus to reduce simultaneously the die area (e.g. with a factor 3) and the current consumption (e.g. with a factor of nearly 70).

Two capacitors (not shown) may be connected across the source-drain path of the transistors T₆ and T₁₀ respectively. They prevent any parasitic simultaneous bias of both nodes N₁ and N₂ to the same value just after the trigger of the comparator, because this would lead to a wrong detection.

Special attention has to be paid to the function of the Schmitt Trigger M₄. This Schmitt Trigger ensures that the comparator is not switched on before the switching voltage to the switch S₈ has reached a sufficiently high value. Consequently it is ensured that the resistive voltage divider is sufficiently “on” when the comparator becomes operative. The Schmitt Trigger also ensures that the switching of the comparator is done in a very short period of time, so that it is immediately fully operative. For example, if the supply voltage is 3 Volts then the switching voltage to the switches S₁ to S₄ will switch from 0 to 3V when the switching voltage to the switch S₈ will have reached almost 3V e.g. 2.5 V.

For similar reasons the Schmitt triggers M₁ to M₃ are introduced. The Schmitt triggers M₁ and M₂ ensure that the nodes N₁ and N₂ effectively reach an equal value before to shut down the comparator. This is an additional security along with the aforementioned capacitors. The Schmitt trigger M₃ ensures that the output at node N₂ is effectively stored in the latch L₁ before the comparator is shut down. 

1. A voltage comparator for comparing an input voltage with a reference voltage, said voltage comparator comprising comparing means (T₁ to T₁₀) and clock-signal controlled switching means (S₁ to S₄) connected to said comparing means for energizing the comparing means upon a transition of the clock-signal and means (M₁, M₂, X, M₃, V) for determining the completion of a comparison and for controlling the switching means (S₁ to S₄) for de-energizing the comparing means when the completion of a comparison is determined, characterized by a voltage divider (D) for dividing the input voltage to be compared, the switching means comprising a switch (S₈) connected in series with the voltage divider (D) for inhibiting the flow of current through the voltage divider when the comparison means are de-energized.
 2. A voltage comparator as claimed in claim 1 characterized by said switching means comprising a Schmitt Trigger (M₄) having an input which is controlled by the signal that controls said switch (S₈) connected in series with the voltage divider, and whose output controls the energizing and de-energizing of the comparing means.
 3. A voltage comparator as claimed in claim 1 characterized by said comparing means having two substantially identical outputs (N₁, N₂), said switching means being arranged to force different voltages to said outputs when the comparing means are de-energized and an exclusive OR gate (X) connected to said two outputs for determining the completion of a comparison when the two output voltages are substantially equal and for controlling, through a clock controlled latch (L₂), the de-energizing of the comparing means and the switching off of the voltage divider. 